The present invention relates to a method for fabricating ceramic articles, and in particular to a method for firing cellular ceramic bodies formed from batch mixtures containing inorganic ceramic powdered raw materials and organic forming compounds.
Popular ceramic multicellular ceramic bodies which are generally formed by extrusion, such as those finding applications in catalytic converters, diesel particulate filters, electrically heated catalysts, and chemical processing, require binders and extrusion aids for proper processing. Typically, water-soluble cellulose ether binders are used; however, such binders alone are not suitable for the manufacture of high cell density cellular structures having a large frontal area and extremely thin walls of less than webs of 0.002 inch (0.048 mm) or less. U.S. Pat. Nos. 6,080,345 and 6,368,992 disclose the use of oils or oil-based compounds, such as polyalphyl olefin, in combination with cellulose ether binders for improved processing of such structures.
While advantageous in providing shape retention for very thin-wall ceramics in the wet green strength, oils lead to significant problems during firing. Specifically, such compounds release small volatile combustibles when heated, which unless controlled effectively can lead not only to cracking of the structural bodies, but also to uncontrolled combustion, or detonation in the kiln. At temperatures above their flash points, the oils decompose quickly and lead to sudden thermal stresses which may increase the risk of cracking in the ceramic articles.
Removal of the organic compounds during firing comprises a sequence of simultaneous reactions which are fairly complex, including, for example, oxidation, volatilization, and thermal degradation. Therefore, it is difficult to remove the organic components from the shaped green ceramic article without incurring distortion or breakage of the part.
Because of the reduced strength of the very twin-wall ceramic honeycomb substrates, and the corresponding increase in the dimensional changes due to the exothermic nature of the removal of the organic compounds, special consideration in the firing must be undertaken to avoid cracking of the ceramic body. Specially designed kilns, apparatus for volatile removal, reduced oxygen containing atmospheres, and complicated firing cycles are among the numerous means that have been employed to control the burnout of organic compounds, decrease the combustible concentrations in the kiln atmosphere, and reduce the thermal stresses differential shrinkage and high cracking frequency. These methods however, require expensive and sophisticated equipment and increase the cost of firing.
In light of the foregoing problems experienced in the art, there remains a need for a method of fabricating and firing ceramic articles processed from batch mixtures containing organic compounds which may include oils or oil-based compounds, which permit safe firing in oxidizing atmospheres with reduced combustible concentrations, to form ceramic honeycomb structural bodies exhibiting less cracks, in a cost-effective and efficient manner.